Coherent optical receivers require adaptive equalizers to compensate for time-varying polarization mode dispersion effects and intersymbol interference (ISI). Probabilistic constellation shaping adds shaping gain that increases the reach of the coherent system and increases the utilization of the communication channel.
Some advances in data throughput in optical receivers rely on the processing of the received and transmitted symbols in blocks rather than serially. However, when block processing is performed, the processing delay in the channel equalizer loop is proportional to block size. The increase in loop delay in the channel equalizer reduces the convergence speed of the equalizer.
Two common types of equalization are generally used in coherent modems: blind equalization which does not require a training stage and has moderate performance, and decision directed equalization which does require a training stage and pre-compensation of frequency and timing offset at the receiver. Some coherent receiver architectures use blind equalization to improve frequency and time offset compensation during the initialization stage after which a switch is made to decision directed equalization. This yields the benefits of decision directed equalization without the need for a training stage.
The performance of the blind adaptive channel equalizers such as Constant Modulus Algorithm (CMA) equalizers deteriorates with Gaussian source signals such as signals with a probabilistic shaped QAM constellation.
An optical transmission channel can have a time varying response because of the rotation in the state of polarization (rSOP) and the polarization mode dispersion (PMD).
In the presence of a shaped constellation, the blind equalization's speed of convergence in reduces. Adding the time variation in the optical channel, blind equalizer convergence becomes challenging, especially for shaped constellations. As a result, the tracking of the channel variations with the blind equalizer needs boosting to reach a convergence state.